By May 7, 2019

The Most ESSENTIAL JavaScript Interview Questions (With Answers!)

Chances are you are preparing for a JavaScript coding interview and now you're faced with preparing for a lot of different JS interview questions.

JavaScript is a high-level programming language, probably one of the most used programming languages in the world right now. It can be used to program the web browser or even servers.

To know the importance of JavaScript, disable JavaScript on your browser and try to load the Web Page in it. Those Web Pages will not work properly. Many contents in them may misbehave.

Usually, JavaScript interview questions are divided into three sections:

Beginner Level
Intermediate Level
Advanced Level

It works to make your interviewer position you inside of a scope of possible choices. The closer you are to an “advanced” level developer, the higher your chances of getting a job.

In today's video, I'll outline the most essential JS questions you should prepare for when it comes to coding interviews and how each one might affect you when it comes to getting you the job or not.

Transcript Of The Video

Jason Humphrey:

In this video, we're going to be talking all about the most essential interview questions for a JavaScript interview. Today I'm going to be taking you through a bunch of questions, some we're going to give you a quick answer. I won't expand on it too much because I have a lot of questions. I encourage you guys all to go Google the essential JavaScript interview questions and study them all up if you have a JavaScript interview coming up soon.

But let's jump into it. I'm going to give you almost like a rapid fire. I'll give a couple of answers here and there, but overall I want to give you the actual questions if you were to come into one of my … It's like a cheat sheet basically. If you were to come into one of my interviews, what I'm going to validate from a JavaScript perspective to make sure that you have claimed or everything validates of what you're claiming to be a great JavaScript developer.

Are you using ES6? Yes, no? If so, yes and why or no and why? I expect an answer either way and I want some really good understanding here of like, yeah, I'm using ES6, which the why doesn't have to be super compelling because ES6 on it's own is very compelling to use for all the different features it provides. If you say yes, though, I do expect your favorite feature and that would be my followup question then. What is your favorite feature you're using? Mine is destructuring, absolutely love it. It makes my life really easy. So what is yours?

Another question I like to ask generally in a coding dow question where I'll give him a fiddle or something to that nature is what is hoisting or I'm going to show them a hoisting problem and just see if they can solve it. With that being said, hoisting is much more advanced and running the hoisting issues is not always common for a lot of people, but it will show me the level of developer you're at.

With the previous question with the ES6 it's kind of like an intermediate. I expect even some beginners can do it but put you up a little bit higher if you can give me that, but this question is much more advanced. A majority of people do not know who hoisting, don't want to answer hoisting, don't care about hoisting and more often than not don't really have to care about it. This type of question will put you higher in the tier or lower in the tier based on your answer. It's okay if you don't get it. It just shows me you haven't gotten to that point in your career yet where you've fully understood the depth of knowledge of JavaScript in all the different problems and pros and cons that come with it.

The difference being between Async and synchronous and showing you a synchronous code and asynchronous code and telling me the difference between them, but I'm going to make them look very similar but I'm gonna want you to tell me which is synchronous, which is asynchronous, because a problem far too often is people write code and think it's going to be synchronous because of how they wrote it, but truth be told it's not. It's asynchronous. Maybe they didn't put a return in. Maybe they didn't use callbacks correctly. There's a lot of things that can go on between async and synchronous code that people can screw up all the time and this is one of the places I like to check to see that you know how to code between the two because it's an easy problem to have and then creates timing issues down the road. That one is a little more advanced too, but this is also where I'm gauging based on these answers I'm going to give you, easier or harder coding question.

Continuing on. When do you use promises verse callbacks or do you just use promises or do you just use callback? Just going straight callbacks is probably more ES5 style within ES6 coming out and so on and so forth to what we're on now with nine promises are absolutely huge. Also the async await, those are becoming huge and what are they using behind the scenes? Promises. But point being, there are still times for callbacks. We still use them but when to use them and how JavaScripts or callbacks verse promises work. It's a large scale question, a lot of knowledge in there, but it's a bit more intermediate to me in my eyes because it's so widely known to use, to use promises nowadays and how callbacks work. If you don't know to call back is, I didn't put this question in here, but I might ask what a callback is and what is the air style for callbacks. I expect that means in your arguments you expect that an error first, results second.

I also expect what are the pros and cons of JavaScript? As you see now with my questions, I'm gauging you. I am gauging you just to know and understand where you're truly at. The pros and cons kind of put your right in the middle and you're able to go up as high as you want or as low as you want because if you use the cop out, we talked about this actually the previous video for no JS. If you're using the easy way out, I drop you at a base level. If you're giving me okay examples and not too much and a couple of facts, you're right in the middle. But if you give me some real world examples, how you used it, how it's benefited you and you just top notch it, this question can really jump you up.

I'm also going to ask this question. I'm going to look and see if you know the difference between global and block level scope. This is a much more basic question. It's if you have a VAR, not even VAR, just you have a let outside of all functions at the very top of the file, that is global.

Another here gimme in the beginning, why do we use use strict and if we don't use it, what could potentially happen? If you can show me a snippet of code that's very beneficial here but it's a basic one. I need you to tell me what Use Strict does. Easiest way for you right now, if you're curious and wondering what is it, well do this. Go into wherever you're writing JavaScript at right now, go put single quote, use space strict, single quote, semicolon at the end, after that line, do a equals single quote, test, single quote and tell me what happens. Because it should error out and it will and if you take Use Strict off and you do a equals test again, it'll automatically work because without strict on JavaScript won't stop you from creating random variables. Use Strict is very beneficial and needs to be and to be able to talk about it because it's something you learn earlier in your career to use.

Another question in the beginning that if you've had some good fundamentals is what is an Iffy? Immediately invoked function expression, very useful in JavaScript check it out there. If you've ever built any jQuery then I care to use in Iffy because Iffy's are very lovely with little jQuery your life.

Here's another question that's going to be more towards the middle. I'm going to want to know what type of JavaScript coding style do you have and how you write it, whether it's the singleton pattern, the factory pattern, the revealing module pattern. I'm not saying you have to know your exact style, but when we talk about me gauging you to see where you're at it's very beneficial. I personally, I like a little singleton. I like a little constructor. I kind of mix my styles based on what I'm doing, but knowing your style is key. If you just write random functions and even then if it's not a specific style like that, give me like do you like smaller functions? Do you like object dory? What are you doing? What's your style that you at least know of? Because if you're just coding stuff just like, oh, give me a problem and you just slapped keyboard a little bit for no reason, it doesn't look that good. It shows me much more beginner level.

Dot verse bracket notation. When do you use one over the other? I see it much more as a basic question because if you want to be dynamic, you need bracket and if you know your variables you can just use dot.

Last but not least, how do you write clean code with JavaScript? How do you ensure your code is readable to someone else? This question I want to see that you are thinking about other people around you with development because if you're going to be joining a team, team aspect comes first. There is no individual in a team. This question is all about how do you think about other people around you? How do you, because when we talk about readable code, you are not going to be the only one reading it. It's pretty inherent when you're in a room of five people getting asked this question or three people, whatever it might be, you're not going to be the only one. How do you make it readable to all of us, because if you can tell us on how you write readable code that now appeals to us, like, oh, if we brought this person on, they could write good code. I'd like this. I'd like reading their code.

Overall, these are my JavaScript must know interview questions that if you were to run into me, I'm probably going to ask you one of these. I highly encourage you to replay this video, take notes, go make sure you answer all the questions. Now I answered some of them for you as we're going through it just to give tidbits, but there's a lot more in depth you can go learn. If you have any comments, questions, please feel free to put them down below and that will do it for us today. Until next time, see you later.

About the author

Jason Humphrey

Jason Humphrey is an full stack development, entrepreneur and investor. He is a professional programmer and engineer working in Node js, Angularjs, HTML5, CSS, JavaScript/jQuery, Mongodb, and Jive. He is a full stack developer, with a special emphasis on and passion for MEAN stack. You can find more about him on his website.